The fault runs for 700 miles off the coasts of California, Oregon, and Washington. But surprisingly, no one knew about the fault line until 45 years ago, Kathryn Schulz describes in her expansive article for The New Yorker.

“The discovery of the Cascadia subduction zone stands as one of the greatest scientific detective stories of our time,” Schulz wrote.

Part of what made discovering it difficult is that the fault doesn’t trigger smaller earthquakes, Schulz writes:

Yet not once in recorded history has it caused a major earthquake — or, for that matter, any quake to speak of. By contrast, other subduction zones produce major earthquakes occasionally and minor ones all the time: magnitude 5.0, magnitude 4.0, magnitude why are the neighbours moving their sofa at midnight. You can scarcely spend a week in Japan without feeling this sort of earthquake. You can spend a lifetime in many parts of the Northwest — several, in fact, if you had them to spend — and not feel so much as a quiver.

But in the 1970s geologists figured out that the fault must exist since it lies right along the newly discovered Ring of Fire — the circle of volcanoes and faults that surround the Pacific Ocean plate:

Starting in the 1980s scientists with the USGS uncovered the key: A “ghost forest” along Washington coast that was wiped out all at once from a rush of sea water. They dated the trees’ death to sometime between August 1699 and May 1700.

Then the nail in the coffin lay in Japan’s record of a mysterious tsunami hitting its shores:

… On the eighth day of the twelfth month of the twelfth year of the Genroku era, a six-hundred-mile-long wave struck the coast, level ling homes, breaching a castle moat, and causing an accident at sea. The Japanese understood that tsunamis were the result of earthquakes, yet no one felt the ground shake before the Genroku event. The wave had no discernible origin. When scientists began studying it, they called it an orphan tsunami.

“The twelfth month of the twelfth year of the Genroku era” happens to be January 27, 1700. Here’s the full story of how those ghost forests were created:

At approximately nine o’ clock at night on January 26, 1700, a magnitude-9.0 earthquake struck the Pacific Northwest, causing sudden land subsidence, drowning coastal forests, and, out in the ocean, lifting up a wave half the length of a continent. It took roughly fifteen minutes for the Eastern half of that wave to strike the Northwest coast. It took ten hours for the other half to cross the ocean. It reached Japan on January 27, 1700.

It’s very likely that this will happen again. The odds that it will come by 2050, Schulz writes, are about one-in-three.